The bluebells have arrived two weeks early. Heavy rain and early sun drew their pale shoots from under warm leaf litter, then the waxen leaves and, at last, the drooping, bell-shaped flower, the yellow anthers and the high, sweet, clear scent.

Late April, early May, before the trees make their lush summer canopy, England's woods are thronged with bluebells. In Heartwood forest, near St Albans in Hertfordshire, they have joined the white wood anemone, the yellow celandines and the tightly knotted ferns that cover the forest floor. "Oh to be in England!" peals a woman passing in a light cagoule. "Now that April's there!"

The English bluebell, not to be confused with the Scottish harebell, is an indicator of ancient woodland, areas where there has been continuous woodland cover since at least the 17th century. Once, Britain was covered by forest like this; "Wildwood" as the ecologist and nature writer Oliver Rackham named it, unsculpted, unplanted, unmanned. Today, ancient woodland makes up just 2% of the UK, having been cleared to make way for farmland, villages, towns, roads and cities. In parts it has been replanted, in the 1970s, with spruce and conifer. We are left with small islands of ancient woodland, fragments of what once was.

Bluebells have a special place in British folklore, their flowers variously believed to prefigure death and to summon fairy gatherings. The gluey sap of their pale white bulbs was once said to cure the effects of snakebite and was used to starch ruffs, bind books and affix feathers to arrows.

In the 1680s, the Spanish bluebell arrived on these shores, introduced as an ornamental garden plant, and celebrated for its hardiness and ability to prosper in any kind of soil. While the English bluebell hangs to one side and droops, the Spanish variety is hardier, scentless and belled on both sides. Their presence, and the possibility of hybridisation, is a major problem for the native bluebell, which is protected by the Wildlife and Countryside Act. However, of all the threats to Heartwood's bluebell population - the spread of the Spanish variety, the foraging of the non-native muntjac deer, the 7,000 new homes promised to St Albans - decimation of its ancient woodland habitat is the greatest.

Heartwood is a project by the Woodland Trust, an attempt to create England's largest new, continuous native forest. It will cover 858 acres. In the parking bay before the scout hut, woodland officer Louise Neicho spreads a map across the bonnet of her car, traces the areas due to be planted with 600,000 native trees - oak, dogwood, ash, maidenhair, hornbeam, walnut. The £8.5m programme began last September, with 170 acres open to the public, who have already come in great numbers, treading fresh pathways, riding horses, walking dogs, heading here for nature days and next week's guided bluebell walks.

Neicho leads us up the hill, past hedgerows that brim and blossom with hawthorn, cow parsley, and flowering nettles, to where the view is of deep green fields that sweep and fold softly into one another, and where a footpath leads off into the darkness of the forest.

There are three pockets of ancient woodland here: Langley Wood, Wells Wood and Puddler's Wood, each quite separate from the next. The forest itself is newly christened, taking its name from the rare species of lime that grows here: its small, heart-shaped leaves opening in early April, a sweet, radiant green. There are oak standards, too, and coppiced hornbeam, planted here long ago to produce charcoal for the bread ovens of London. The cherry trees, common to this corner of Hertfordshire, are in full blossom, and the pathway is speckled with their pale pink petals, twigs, hazel burrs, and dappled sunlight.

In a clearing between the hornbeams and the limes, the bluebells sprawl in a haze of colour, of lilac blue and, in the shadows of the trees, a deeper, violet shade. It is quiet here, save for the low drone of a fat bee, the rat-a-tat-tat of the woodpecker, and the far-off clatter of the high-speed train to the city. The bluebells' scent, what Gerard Manley Hopkins called "the faint honey smell", rises up past the spider webs that hang on the branches, buoys a large white butterfly, brushes the soft green leaves, touches the pale blue sky.